Luke Shaw is set for a spell on the sidelines after Manchester United confirmed the left-back would miss the start of the Premier League season with a hamstring injury.
Shaw, who joined the club from Southampton in June for £27million, faces a four-week lay-off, meaning he will miss United's opening league game against Swansea on Saturday.
The 19-year-old, who became the world's most expensive teenage footballer when he made the move to Old Trafford, could also miss further Premier League games against Sunderland and Burnley.
Shaw made his United debut against LA Galaxy in July during their summer tour of North America.
Earlier on Wednesday, the Red Devils confirmed that Shaw had been given the number three shirt for the forthcoming season, after it had been vacated by Patrice Evra's departure for Juventus.
United are currently trying to sign Sporting Lisbon defender Marcos Rojo as cover for Shaw.
Reports suggest the Red Devils have made a bid of around £16million for the 24-year-old Argentina international and that Rojo has refused to train with Sporting, handed in a transfer request and asked to be allowed to join United.
Sporting president Bruno de Carvalho has been quoted as saying that both Rojo and team-mate Islam Slimani are "subject to strong disciplinary measures" and will not play in the club's Portuguese league opener against Academica on Saturday.
De Carvalho said: "They will not play next weekend.
"This will take as long as it takes. If something doesn't change, you're going to end up with the players seeing out their contracts.
"They can watch the game on television.
"On Saturday, against Academica, we will play to win and we will win.
"There is no player bigger than the club. I would not give up the honour, the club's history, for the attitudes of anyone.
"I give a piece of advice: don't use the press, be professional.
"These two, in defence of our magnificent group, must be removed. They are subject to strong disciplinary measures.
"Both players have taken decisions that have caused sharp indiscipline at Sporting.
"A message: any player who does not follow the rules of the club can have illusions and pretensions, but with the way these two did things, the situation is just complicated."
Meanwhile, United have announced their squad numbers for the new season, with Adnan Januzaj inheriting the shirt previously occupied by Ryan Giggs.
Belgium midfielder Januzaj, also 19, takes the number 11 famously worn during his illustrious playing career by Giggs, who is now Van Gaal's assistant.
Ander Herrera, the midfielder signed this summer from Athletic Bilbao, has taken number 21 in place of Angelo Henriquez, departed to Dinamo Zagreb on loan.
Juan Mata has kept the number eight shirt, with fellow midfielder Anderson being demoted to 28.
No-one has claimed the number five or 15 shirts worn previously by Rio Ferdinand and Nemanja Vidic, the centre-backs now with QPR and Inter Milan respectively.